1. Field of the Invention
The present invention relates to an image sensor incorporating an A/D conversion circuit, and an image sensing system.
2. Description of the Related Art
Japanese Patent Laid-Open No. 2012-060334 discloses a technique of saving an error when a defect bit is generated in a data holding unit in a CMOS image sensor incorporating an A/D conversion circuit.
In the above patent literature, however, to specify the defect bit in the data holding unit, special driving of the data holding unit is executed at a timing other than a normal image sensing timing. That is, an operation for examination is required.
Even if a normal state is determined when executing driving for specifying the defect bit, if a defect bit is generated after that, the defect bit cannot be saved. For example, if information of a defect bit is acquired by executing driving for specifying the defect bit only upon shipping from a factory, when a new defect bit is generated due to age deterioration or electrical/physical damage after shipping, an error cannot be saved. Alternatively, if information about a defect bit is acquired by executing driving for specifying the defect bit at a given timing, for example, upon power-on, when a defect bit dependent on the temperature is generated during normal image sensing, an error cannot be saved. Also, when a defect bit is suddenly generated only at a given timing like a software error, the error cannot be saved. The present invention has as its object to provide an image sensor that can specify a defect bit in data and recover an error even if the defect bit is suddenly generated in the data in the image sensor during image sensing.